Casting metal powders



United States atent hFice 3,115,698 ATlNiG METAL PGWDERS Philippe D. 55.St. Pierre, Schenectady, N.Y., assign'or to General Electric flompany, acorporation of New York No Drawing. Filed Feb. 25 E60, Ser. No. 11,483 1Claim. it'll. 29--182) This invention relates to a method for formingmetal articles having relatively complex shapes and more particularly toa method and composition providing for slip casting of metal powder toproduce green ware of good strength and uniformity.

This application is a continuation-in-part of copending applicationSerial No. 751,901, filed July 30, 1958, now abandoned, and assigned tothe same assignee as the present invention.

Due to the requirements of present-day technologies, an increasingnumber of metals and alloys are being developed which possess specialmechanical and thermal properties rendering them particularly applicablefor use in special situations. However, since these metals and alloysmust very often be formed into selected and often elatively intricateshapes, the ease with which they can be fabricated becomes a materialconsideration. Some metals, such as molybdenum and tungsten, amongothers, are diiiicult to work and therefore require special processing.Even then, the production cost is high and the reject rate is above thatof more easily worked materials, this feature further increasing theunit cost of the final product.

Although some fabrication diiiiculties have been overcome by resortingto powder metallurgy techniques, in general the shapes which can be madeby this process are restricted. This restriction results, at least tosome extent, from the fact that high compacting pressures are normallyused and the receptacle holding the metal powder must be able towithstand the forming pressures.

As a possible solution to the problem of shaping powder metals intointricate shapes, it was attempted to fluidize metal powders and castthem in a manner analogous to slip casting. Due to various factors,however, this method has not been adopted because of its unreliabilityin producing strong, sound green castings for subsequent firing intometal articles.

It is a principal object of this invention to provide a process and acomposition for slip casting metal powder.

It is another object of this invention to provide a compositioncontaining finely divided metal powders which can be used in a slipcasting process to consistently produce articles of good green densityand strength.

Still another object of this invention is to provide a process and aslip composition for forming articles of molybdenum and/ or tungstenpowder.

Other objects and advantages of the present invention are in partobvious and in part explained in the accompanying specification.

Briefly, the present invention comprises a process for slip castingmetal powders, and more particularly refractory metals such asmolybdenum and tungsten, and utilizes a slip composition comprising afluidized metal powder, a viscosity controlling addition, andanti-foaming agent to reduce the formation of bubbles, and an additiveto promote uniform wetting of the metal particles, the lastrnentionedfeature being important from the standpoint of obtaining reproducibleresults.

Several factors must be considered in preparing a slip compositioncapable of use for the casting of metal powders into preselected shapes.For example, the solid-toliquid ratio or" metal to liquid, the type andparticle size of metal which is being cast, the nature of the oxidecoating on the metal, the acidity or basicity of the slip, and thefoaming characteristics of the final composition.

Patented Dec. 31, 1 363 These factors are discussed more fully later andalthough the discussion is directed primarily toward the slip cast in ofmolybdenum and tungsten, it is to be understood that other metals andalloys having similar properties can be cast in the same way.

Considering each of the various factors mentioned in the precedingparagraph, the particle size of the powder metal particles is ofparticular importance, since it de termines whether the system iscontrolled principally by physical considerations. f the particle sizeexceeds a certain maximum, the system is regulated principally on thebasis of physical phenomena, whereas if this maximum size is notexceeded, chemical considerations prevail and the ionic atmosphere ofthe slip is important. That is, the rheology of the slip is determinedprincipally from the standpoint of the chemical and ionic conditionsrather than by the effects of gravity.

In preparing a slip of either molybdenum or tungsten powder, forexample, it has been found that about percent of the powder should be of10 micron size or smaller. If the percentage of particles greater than10 micron size exceeds 90 percent, then the system is controlledprimarily by gravitational considerations rather than by chemicalconsiderations. Of the 90 percent of the particles which fall within therange of from O to 10 microns, at least 50 percent of these particlesshould be less than 5 microns in particle size, although if all of theparticles are within the 10 micron range, the chemical properties of thesystem will still control. The metal powders are preferably dry milledin a porcelain ball mill using porcelain balls and in an inertatmosphere or inert liquid environment which precludes any unnecessaryoxidation of the particles. Milling in water or other oxidiz ingenvironment promotes oxidation, this oxide then dissolving in the slipliquid and causing the slip to sit up and gel after a short time.

The finely divided molybdenum or tungsten powder is then mixed with asuitable volume of liquid to render it sufilciently fluid to enablepouring into an absorbent mold, a mold similar to that ordinarily usedin slip casting processes being acceptable. It is desirable to make thepowder-liquid mixture as dense as possible consistent with pourability,preferably so that there is a volume ratio of from 1.25 to 2.5 parts ofliquid to one part of solid, although as much as 5 parts of liquid toone part of solid can be used. The liquid referred to includes not onlywater but also all of the additional liquid modifying agents used in theslip. Use of large quantities of liquid can increase the expense of thecasting operation both by requiring additional materials and bynecessitating a longer time for removal of the liquid into the porousmold. The small particle size of the metals, in addition to its otherrequirements and advantages, permits the use of quantities of liquidssmaller than could be used with larger particles.

Another feature which must be considered is the hydrogen ionconcentration of the mixture. When casting finely divided metal powders,it has been found that the pH should fall within the range of from 3 to9. For molybdenum powder, for example, the preferred pH is from 6 to 7,whereas for tungsten powder, a pH of about 4 to 6.

The pH of the system is felt to have some effect upon the quality of thefinal green casting primarily in its effect upon the oxide coatingcontained on the individual powder particles. Control of pH is readilyeiiected through the use of suitable acid or base additions. In thepresent case, hydrochloric acid is used to increase the acidity or" theslips while ammonium hydroxide has been found eiiective in increasingthe basicity of the slips. In an extremely fine material of the typebeing used in the present invention, the ratio of oxide coating toparent metal is much greater than it is where the particles are largeenough to be primarily controlled be gravity. Therefore, the amount ordegree of hydrolysis of oxide coating has an effect upon the quality ofthe casting obtained and upon the reproducibility of results. That is,if the oxide coating does not hydrolyze to the same degree in each batchof slip, the resulting green castings will vary. Therefore, one of thefactors which controls the hydrolyzation of the oxide coatings is the pHand strict control over this will assist in achieving reproducibleresults.

The apparent viscosity of the slip should approximate the viscosity of aliquid such as glycerin at room temperature, this being the usualviscosity of mixtures used in slip casting. It has been found that a lto 5 percent aqueous solution of polyvinyl alcohol will form a suitableliquid vehicle at the proper viscosity for pouring. Other recognizedviscosity-controlling agents such as sodium or ammonium alginate ormethyl cellulose may be used to locate the viscosity of the slip withinthe proper range.

Also important to the attainment of a proper casting is the inclusion ofa suitable antifoaming agent such as one of the silicones, octyl alcoholor amyl alcohol. Other and well known antifoamants may also be used. Thefunction of the antifoaming agent is to prevent the formation of airbubbles within the slip which would present the possibility of airbubbles being trapped in the green ware. It air bubbles are trapped inthe wall of the articles being cast, it is obvious that the finalproduct will be structurally weak. The amount of antifoamant used in agiven situation will vary, since the removal of foaming is essentially asurface reaction. For example, a slip held in a shallow containercausing a large surface area to be present will require more antifoamantthan the identical slip in a deep container causing a comparativelysmall surface area of the slip to be exposed. Also, when a quantity ofslip is poured from a container into a mold, a great deal of theantifoamant is lost from the surface of the slip so that additionalquantities may have to be used.

The final agent used in the present slip composition is a wetting agentsuch as aerosol OT which is dioctyl sodium sulfosuccinate, a wettingagent produced by the American Cyanamid Company. The aerosol OT is notthe only wetting agent which can be used, although it is an extremelyeffective one in the refractory metal systems and preferred for thatreason. Other sulfonated organic wetting agents may be substituted forit. The wetting agent insures that each of the individual particles willbe uniformly wetted so that from batch to batch the same degree ofwetting will always occur, thus insuring reproducibility of results.Generally, it has been found that a small but effective amount, between0.01 to 0.50 gram of the wetting agent per 100 cubic centimeters ofmetal in the slip will ensure the proper degree of wetting, with anaddition of 0.1 gram being preferred in most instances. The use of thewetting agent assists uniform hydrolyzation of the oxide coating in eachinstance.

As an example, a molybdenum slip made according to the present inventionwas prepared by mixing 125 cubic metal bodies.

centimeters of 3 weight percent aqueous polyvinyl alcohol with 100 cubiccentimeters of molybdenum, the molybdenum particle size all being withinthe 10 micron limit. A small amount of antifoam was added, the amountnot being critical, and 0.1 gram of wetting agent (aerosol OT) was alsointroduced. The entire mixture was poured into an absorbent mold andwhen the desired wall thickness was reached, the remaining slip waspoured out. The resulting green ware was found to have excellent greenstrength and density. Upon firing at 1900 C. in hydrogen, the resultingbody was not only substantially free from pores but also presented apleasing appearance and had excellent mechanical strength.

A tungsten slip was also made according to the invention by mixing 100cubic centimeters of tungsten powder of which at least 90 percent wasless than 10 micron particle size with 200 cubic centimeters of 3 weightpercent polyvinyl alcohol solution. To this was added about 0.1 gram ofwetting agent (aerosol OT) to insure uniform hydrolyzation of thepowder. The pH, which was initially slightly in excess of 5, was reducedto 4.5 through suitable addition of hydrochloric acid. This mixture wasthen poured into an absorbent mold, the liquid withdrawn, and theresulting green body fired similarly to the molybdenum powder previouslydescribed to produce a body of uniform quality and excellent mechanicalstrength.

Thus, the present invention has described a method and a compositionwhereby metal slips can be prepared and cast to make bodies havingsufiicient green density to form As already mentioned, the variousfactors can be controlled to suit the requirements of similar metals.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

A slip composition of pH of from about 3 to about 9 for forming metalpowder castings having high green strength and substantially uniformthickness, said slip consisting essentially of metal powder at least 90per- ,cent of which does not exceed 10 micron particle size,

from about 1.25 to 5.0 parts of a water carrier for each part of metalpowder to fluidize said metal powder for pouring into an absorbent mold,an amount of polyvinyl alcohol dissolved in said liquid carriersuflicient to form a l to 5 weight percent solution, an antifoamingagent selected from the group consisting of silicone antifoamants, amylalcohol and octyl alcohol to reduce formation of bubbles and assistproduction of a sound casting, and from about 0.01 to 0.50 gram of asulfonated organic wetting agent promoting uniform hydrolyzation of saidmetal powder and improving the rheological characteristics of the slip.

References Cited in the file of this patent UNITED STATES PATENTS

